Bootstrap Grid System and many other responsive CSS frameworks used media query to detect the screen size. Media query is calculating the screen size based on its resolution, how much pixels available on the screen, not the actual size of the screen IRL.

If you are just downsizing the window without using the device toggle, you don't know exactly how much pixels are there in the window.

So what I suggest is to use the device toggle and adjust the pixel size based on your need if the device is not available in the option.